1. Field of the Invention
The present invention related to a method for calibrating a breathing monitor which measures respiration volume by separately measuring and then summing the contributions from rib cage and abdomen plethysmograph sensors.
2. Description of the Related Art
A prior art procedure for calibrating breathing monitors with transducers placed on both the rib cage and abdomen is disclosed in U.S. Pat. No. 4,834,109, the entire contents of which is incorporated herein by reference. This procedure is referred to as Qualitative Device Calibration (QDC) and includes the step of determining a ratio (K), also referred to as the scaling factor, of the gains of the rib cage and abdominal transducers during natural breathing for reflecting the relative contributions of the rib cage and abdomen of a patient or subject to the respiration volume. During the calibration procedure, the tidal volume is made equal to the sum of the weighted rib-cage and abdomen waveforms during the calibration period and is designated as a baseline which is used to assess changes in tidal volume as a percent of the baseline. The tidal volume can be converted to an actual volume by breathing into a volume measurement device and obtaining a conversion factor designated as a multiplicative factor (M).
After the QDC procedure is completed, with or without the computation of M, the breathing monitor is capable of detecting obstructive apneas in which the upper airways become occluded while the subject makes respiratory efforts. Since the rib-cage and abdomen transducers are calibrated, the occlusion causes a flat or near flat sum of these two signals to be generated, indicating that there is no tidal volume. This condition may thus be diagnosed as an apnea or cessation of breathing.
Changes in breathing patterns of the subject and/or changes in the posture of the subject may affect the scaling factor K, causing shifts in the contributions of the rib-cage and abdomen to tidal volume and thereby causing inaccurate estimates of tidal volume. To counter this effect, the scaling factor of the gains K may be recomputed at different time intervals using the QDC procedure to ensure that the scaling factor matches current conditions. The multiplicative factor M, which corresponds to the initial calibration breathing pattern and posture, may also be affected by changes in posture and breathing patterns. Although the recalibration of M may also be accomplished, such recalibration requires that the subject periodically breath into an external measurement device. This somewhat conflicts with the purpose of the breathing monitor which is the long term, non-invasive respiratory monitoring.
Another prior art procedure for calibrating breathing monitors with transducers placed on both the rib cage and abdomen is disclosed in U.S. Pat. No. 4,373,534, the entire contents of which are expressly incorporated herein by reference. This reference discloses the least squares, linear regression, or multi-linear regression methods for calibrating respiratory inductive plethysmograph devices while simultaneously breathing into an external measurement device. At least two sets of rib-cage and abdomen measurements and breathing volume measurements are taken with each set having different ratios of rib-cage and abdomen contributions to tidal volume xe2x80x94i.e., different scaling factors. The two sets of readings or measurements are then used to determine the average scaling factor of the rib-cage and abdomen sensor gains and to determine the optimal multiplicative factor M. Since this method changes the rib-cage and abdomen contributions to tidal volume in each set of readings, this method yields an average value for both the scaling factor K and the multiplicative factor M. This method provides a better estimate of actual tidal volume during various postures and breathing patterns of the subject. However, it is more difficult to diagnose obstructive apneas using this method because this method uses average values and not actual values of the scaling factor K.
It is an object of the present invention to provide a calibration procedure for a non-invasive tidal volume measurement device which uses the two compartmental lung model which provides an accurate estimate of tidal volume without limiting the ability of the device to detect obstructive apneas.
A method for calibrating a non-invasive breathing monitor having a rib-cage sensor and an abdomen sensor according to an embodiment of the present invention, comprises the steps of determining an optimal multiplicative value for the rib-cage sensor and abdomen sensor readings via one of least squares, linear regression, and multi-linear regression techniques. A current scaling factor is then determined using a Qualitative Device Calibration procedure for the current posture and breathing pattern of the subject. The current scaling factor and readings from the rib cage and abdomen sensors are used to accurately diagnose obstructive apneas. The optimal multiplicative value, the current scaling factor, and readings from the rib cage and abdomen sensors are used to determine the current tidal volume.
The current scaling factor may be determined on a periodic basis based on a fixed time period or based on a moving window average related to a number of breaths.
The step of determining an optimal multiplicative value also includes determining a scaling factor which is used for determination of the multiplicative factor. At least two sets of readings of the rib-cage and abdoman sensors are taken, with each set having different ratios of rib-cage and abdomen contributions to the tidal volume. Therefore, the optimal multiplicative is an average multiplicative for the various sets of readings.
The scaling factor used for determining the optimal multiplicative may be used for diagnosing apneas, particularly if the subject has not changed positions or breathing patterns. However, in the preferred embodiment, the current scaling factor is used.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.